Micromachined devices are typically formed by either bulk micromachining techniques or surface micromachining techniques. When using bulk micromachining, a single-crystal silicon substrate is used to create one or more components, and the components are joined to form a micromachined device. Each component typically has a thickness in the range of several hundred microns, and may have lateral dimensions ranging from tens of microns up to several millimeters. The micromachined device components may be formed by etching a single crystal silicon substrate wafer, using either wet etching (such as anisotropic etching) or dry etching (such as reactive ion etching, or "RIE") to form the features. However, micromachined devices formed of a substrate of single crystal silicon have an inherent propensity to fracture due to the aligned crystal structure of the substrate material. Single crystal silicon tends to fracture relatively easily, and the fractures propagate along the planes of the crystal structure. As a result, the prior art bulk micromachined structures are prone to fracturing and cleaving during assembly or operation of the micromachined device.
Accordingly, there is a need for a method for forming bulk micromachined components of a material having improved fracture-resistant qualities. There is also a need for a method for forming micromachined device components out of single crystal silicon substrates, wherein the resultant micromachined device structure has improved fracture-resistant qualities.
The present invention is a method for forming micromachined devices from a polycrystalline silicon substrate using deep reactive ion etching to form the micromachined device. The resultant micromachined device structure is formed entirely of polycrystalline silicon, thereby offering superior fracture resistance. Polycrystalline silicon does not have the aligned crystal planes of single-crystal silicon, and thus the resultant micromachined device is not as susceptible to fracture.
Relatively recently, deep reactive ion etching (DRIE) has been increasingly used with single crystal silicon substrates to form micromachined devices. DRIE offers highly directional etching, and is not crystal plane dependent. Applicants' invention recognizes the benefits of combining DRIE with the use of a polysilicon substrate to form micromachined devices formed out of a bulk polysilicon substrate.
DRIE may be used with a single crystal silicon substrate having a polysilicon top layer deposited upon the surface of the silicon substrate, as discussed, for example, in U.S. Pat. No. 5,438,870. In such use, the polysilicon top layer typically ranges in thickness from a few microns up to tens of microns, and is deposited upon a single crystal silicon substrate. A layer of silicon dioxide may be grown or deposited on the substrate before the polysilicon surface layer is deposited thereon. Surface micromachining is then used to etch the substrate. In surface micromachining, the etching techniques require and use the differing layers within the substrate to achieve the desired results. For example, as mentioned above a substrate may be formed having a layer of polysilicon on top of a layer of silicon dioxide, which is in turn on top of a single crystal silicon wafer. Surface micromachining etching techniques may then be used to etch the substrate, and these techniques typically rely upon the chemical properties of each of the differing layers to aid in the etching. For example, one step in the etching process may consist of placing the substrate in a solution that dissolves silicon dioxide, but does not dissolve the layers of single-crystal silicon or polysilicon.
This is to be contrasted with bulk micromachining, which is a separate and distinct technology. In bulk micromachining components are formed by deep etching into the body of the wafer, and the etching does not rely upon differing layers in the substrate. Instead, the desired shape is formed directly in the substrate wafer. Bulk micromachining also requires differing etching tools and techniques because the depth of the subject material differs by at least an order of magnitude than that used in surface micromachining. Atomizers, pressure sensors, accelerometers, rate sensors and ink jet printheads are all examples of devices that can be formed by bulk micromachining.
In another embodiment of the invention, two or more layers of single-crystal silicon are joined together such that their crystal planes are misaligned, and the resultant composite is used as a substrate for forming micromachined devices. In the prior art, when substrates have one or more layers of single crystal silicon, the substrate layers typically are arranged such that their crystal structures are aligned. Silicon wafers typically have a feature, such as a flat, formed in the wafer which identifies the crystal orientation of the wafer. During manufacturing, the flats of stacked wafers are aligned for ease of processing the stacked wafers. However, the resultant structure is susceptible to fracture, as the aligned crystal planes allow a fracture to propagate through the entire micromachine structure. The present invention utilizes misaligned crystal layers to inhibit fracture propagation through the micromachined device.